1. Field of the Invention
The invention relates to a printing blanket used for an offset printing press and more particularly to a printing blanket having a compressive layer.
2. Description of the Related Art
In an offset printing press, an image of a lithographic plate is printed on a paper through a blanket, and after the image of the lithographic plate is copied on the blanket, the image on the blanket is printed on the paper. In such a blanket used for an offset printing press, there is a compressive blanket having a compressive layer with a porous layer.
Based on FIG. 2, an example of a compressive blanket will be explained below. The compressive blanket comprises a reinforcement layer 3 laid in two to three layers of fabrics 1 such as cotton cloth, rayon cloth, and polyester cloth through adhesive layers 2 such as rubber cement, a compressive layer 4 which is a fine porous layer formed by foaming of a foaming agent on the reinforcement layer 3, a supporting body 5 made of cotton cloth laid on the compressive layer 4, and a surface rubber layer 6 laid on the supporting body 5. Coater such as knife coater and blade coater is used to lay said surface rubber layer 6 on the supporting body 5.
An object of having a compressive layer is to prevent a blur that a picture image becomes unclear when uneven pressure is applied on a printing surface. Another object is to cushion and absorb the shock that is applied when two or more sheets of paper are accidentally inserted during the printing process, to absorb damage of the blanket, or to protect the blanket from damaging the printing quality. Another purpose is to maintain the flatness and thickness of the printing surface by restoring the blanket compressed in the nip portion of a printing machine to the normal thickness. A compressive printing blanket is used for a high speed offset printing press to accomplish such purposes as well as obtain clear printing images.
However, as mentioned above, even though a blanket has a compressive layer, the change in pressure cannot be absorbed completely, creating a streak defect (shock eye) and a smash trouble. A streak defect is a defect, which creates a horizontal stripe on a printed material because the printing pressure changes with a rapid change in pressure and vibration caused when a cylinder gap section of a printing machine passes through a nip. It is also called a shock eye because a streak defect is that a shock generated in a printing machine affects a printed material. A smash trouble is also a defect, which dents a blanket because when a printing paper is tore during the printing process, and two or more papers go into the nip, a compressed blanket cannot be restored to the normal thickness. A blanket with low compressibility is used to aair space a streak defect (shock eye) and smash trouble. However, although a streak defect (shock eye) is somewhat improved by using a blanket with low compressibility, the ink transition pressure between a printing cylinder and a blanket drum and between a blanket drum and an impression cylinder (pressure between nips) declines, deteriorating printing quality (poor ink impression). Moreover, even if a blanket with low compressibility is used, the compressive amount (the amount of air space) of a conventional compressive blanket is limited, and it cannot respond to a smash trouble when excessive printing pressure is momentarily applied, thereby denting the blanket. In this specification, the amount of air space means the total of the thickness of air space occupied to a vertical section in a compressive layer.
Therefore, it is an object of the invention to provide a printing blanket designed to reduce a streak defect (shock eye) while maintaining printing quality. It is also an object of the invention to provide a printing blanket in order to reduce a smash trouble while maintaining printing quality.